Those skilled in the art of practically useful holograms have for some time made considerable efforts to attain high diffraction efficiency and excellent durability, i.e. to obtain holograms that do not change due to exposure to light or variation of humidity. Studies have been carried out in this area as relates to recording materials for holograms and treatment thereof. Thus, many recording materials such as silver salt photosensitive materials, photoresist, thermoplastic resins, photopolymers or bichromic acid-gelatine, etc. and methods of treating thereof have been examined. Among these arts, it has been known that holograms obtained by bleaching a silver salt photosensitive material and holograms using bichromic acid-gelatine have a diffraction efficiency of higher than 50% when they are examined just after production thereof. Regretfully, however, these holograms have problems because the gelatine layer composing the holograms swells or contracts by variation of humidity to cause deterioration of the diffraction efficiency or variation of the optimum reproducing condition. Furthermore holograms obtained by bleaching silver salt photosensitive materials have problems that, since they contain silver compounds, the compounds are printed out to cause coloration upon exposure to reproduction light such as laser rays, etc., resulting in deterioration of the diffraction efficiency. Under existing circumstances, a maximum diffraction efficiency of 15% can be obtained when using the prior recording materials except for the silver salt photosensitive materials and the bichromic acid-gelatine materials.
There have been attempts at improving the durability of holograms. However, processes for producing glass holograms having an uneven surface are comparatively new arts. One of the process for producing glass holograms has been described in U.S. Pat. No. 4,056,395. According to this process, uneveness corresponding to a hologram pattern is formed on the glass base by forming a metal silver image corresponding to the hologram pattern on a recording material comprising a silver halide photographic emulsion layer provided on a glass base and thereafter carrying out ion-etching. In addition, glass holograms can be produced by a process which comprises forming a relief image of a photoresist corresponding to a hologram pattern on a recording material comprising a photoresist sensitive layer provided on a glass base or forming an image of refractivity of gelatine corresponding to a hologram pattern on a recording material comprising a bichromic acid-gelatine sensitive layer provided on a glass base and subsequently carrying out ion-etching to form uneveness corresponding to the hologram pattern on the glass base (see, for example, U.S. Pat. No. 3,733,258).
The holograms produced as described above are comprised of only glass. Accordingly, durability which is required in holograms obtained by bleaching silver salt photosensitive materials or holograms using bichromic acid-gelatine is improved. However, these glass holograms are not desirable because the diffraction efficiency cannot be increased too much, and the diffraction efficiency obtained in an example described in U.S. Pat. No. 4,056,395 is at most 15%.
Therefore, high diffraction efficiency and excellent durability are not both attained by the above described hologram recording materials and methods of treating thereof. This fact is a serious obstacle to practical use of the holograms.